Intrafusal fibers of muscle spindles are innervated in the central region by afferent sensory axons and at both polar regions by efferent γ-motoneurons. We previously demonstrated that both neuron–muscle contact sites contain cholinergic synapse-like specialisation, including aggregates of the nicotinic acetylcholine receptor (AChR). In this study we tested the hypothesis that agrin and its receptor complex (consisting of LRP4 and the tyrosine kinase MuSK) are involved in the aggregation of AChRs in muscle spindles, similar to their role at the neuromuscular junction. We show that agrin, MuSK and LRP4 are concentrated at the contact site between the intrafusal fibers and the sensory- and γ-motoneuron, respectively, and that they are expressed in the cell bodies of proprioceptive neurons in dorsal root ganglia. Moreover, agrin and LRP4, but not MuSK, are expressed in γ-motoneuron cell bodies in the ventral horn of the spinal cord. In agrin- and in MuSK-deficient mice, AChR aggregates are absent from the polar regions. In contrast, the subcellular concentration of AChRs in the central region where the sensory neuron contacts the intrafusal muscle fiber is apparently unaffected. Skeletal muscle-specific expression of miniagrin in agrin−/− mice in vivo is sufficient to restore the formation of γ-motoneuron endplates. These results show that agrin and MuSK are major determinants during the formation of γ-motoneuron endplates but appear dispensable for the aggregation of AChRs at the central region. Our results therefore suggest different molecular mechanisms for AChR clustering within two domains of intrafusal fibers.
In muscle spindles, cholinergic synapse-like specialisations (including aggregates of AChRs) form in the central and polar regions of intrafusal fibers. We analysed the role of agrin and its receptor complex during muscle spindle development. We found that in E18.5 agrin- and MuSK knockout animals cholinergic synapse-like specialisations are present in the central region, whereas they are absent in both polar regions. These results suggest different mechanisms for AChR aggregation within both regions of intrafusal muscle fibers.